1. Field of the Invention
The invention relates to a peak detecting apparatus and, more particularly, to a peak detecting apparatus for detecting peak values of signals from a plurality of signal sources.
2. Related Background Art
Hitherto, as an apparatus for detecting peak values of signals from a plurality of signal sources, a circuit shown in FIG. 10 has been proposed (Japanese Patent Application No. 7-167407 or the like).
In FIG. 10, IN.sub.1 to IN.sub.3 denote input terminals which are connected to signal sources. The input terminals are connected to holding capacitors CT.sub.1 to CT.sub.3 and Darlington type emitter-follower circuits Q.sub.11 .multidot.Q.sub.21 .multidot.M.sub.31 to Q.sub.13 .multidot.Q.sub.23 .multidot.M.sub.33, which are buffer means, through switching means M.sub.21 to M.sub.23, respectively. M.sub.11 to M.sub.13 denote reset means. Output sides of the buffer means are commonly connected and output signals are increased by proper gain times by an amplifier A.sub.1 and are outputted.
The above circuit, however, has a problem such that output values differ in dependence on the number of signal sources for generating peak values. When explaining it by using numerical expressions (it is now assumed that NPN transistors which are Darlington connected are set to one NPN transistor). Now, assuming that
the number of signal sources =N PA1 the number of signal sources for generating the peaks =NP PA1 peak voltage value =VP PA1 saturation current of the NPN transistor =IS PA1 constant current value of the buffer means =I,
a peak output voltage is EQU V.sub.OUT =VP-kT/q.multidot.ln(N.multidot.I/NP.multidot.IS)
As will be obviously understood from the above equation, in the foregoing circuit, there is no problem in the case where the signal, namely, VP (the first term of the above equation) is sufficiently larger than that of the second term. However, when the VP is small and the second term cannot be ignored, there is a problem such that as the number of signal sources for outputting the peak values is small, the output voltage decreases. A detecting precision is deteriorated.